Flat panel displays are used to display video information in a variety of applications, laptop computers, for example. While flat panel displays can advantageously provide a compact form factor, flat panel displays, like nearly all electronic systems, generate electromagnetic interference ("EMI"). Because EMI adds signals to an already congested radio spectrum, the amount of permissible EMI is subject to applicable governmental regulations.
The EMI-radiating performance of a system may be evaluated by measuring equipment emissions within a narrow frequency reference window at individual frequencies. In the United States, applicable Federal Communications Commission regulations dictate using a 120 KHz wide (e.g., f.sub.m =120 KHz) standard reference measurement window that is swept from about 30 MHz to 1 GHz for purposes of making EMI measurement. Measurement involves a time integration of the spectral energy of the emissions occurring within the reference measurement window. The measured average emission magnitude at each frequency window is compared to published pre-specified limits, and a determination is made as to whether excessive EMI is being radiated. If excessive radiation is present, measures must be taken to bring the EMI-emitting system into compliance within acceptable emission limits.
It is known in the prior art to absorb or otherwise attenuate emitted EMI. It is also known to generate signals having less spectral energy that falls within the bandwidth of the EMI reference measurement window. These prior art techniques will now be described with respect to reducing EMI in a video display system.
FIG. 1 depicts a video flat panel display 10 and its display generator system 20, as well as several prior art techniques commonly used to reduce EMI-emissions 30 from the video flat panel display.
Display generator system 20 comprises a main oscillator 40 whose frequency is normally crystal controlled. The frequency of the main oscillator output signal is reduced by a frequency divider 50 and is then provided as input to a timing generator 60. Timing generator 60 further divides the main oscillator clock signal to generate a lower frequency panel clock signal 70. This panel clock signal is used to clock pixel brightness data out of a video frame buffer 80 via a data bus 90 to the flat panel display. The frame buffer data may be a single bit, or an entire word of data whose bits are clocked simultaneously.
Timing generator 60 also produces horizontal and vertical synchronization signals, 100, 110. These synchronization signals permit video panel display 10 to align incoming data received via data bus 90 with a particular (x, y) location on the display panel. As such, flat panel display 10 has no internal clocks or other time dependent element, and has no inherent time dependencies.
Data carried on bus 90 is displayed sequentially on flat panel display 10, with the displayed position of each pixel being determined by the number of clock pulses from a reference synchronization signal. In alternative implementations, the horizontal and vertical synchronization signals are replaced with explicit address lines to locate specific pixel positions. Such implementations permit data to be displayed in a more random fashion, somewhat analogously to accessing data within an integrated circuit random access memory.
Referring briefly to FIG. 2A, the panel clock signal typically is a periodic square wave pulse train, with a repetition frequency f.sub.c of about 5 MHz, and rise and fall transition times on the order of 2-4 ns. In most applications, the pixel data from frame buffer 80 is clocked over the data bus 90 to the flat panel display 10 on each rising edge of the panel clock signal. As shown, the rising edge of each panel clock signal is equidistant in time from the previous rising edge.
FIG. 2B is a frequency domain representation of the frequency spectra of the panel clock signal, which is to say the Fourier transform of the corresponding square-wave panel clock signal. Because the panel clock signal has relatively fast rise and fall times, the corresponding spectral amplitude will be rich in harmonics, centered about odd multiples of the base frequency f.sub.c. Shown in phantom in FIG. 2B is the bandwidth of the reference window used for EMI-compliance testing. Because of the rapid 2-4 ns transition times, the time domain waveforms of FIG. 2A will, unfortunately, be rich in EMI. As a result, as the EMI standard reference window sweeps back and forth horizontally, along the frequency axis, there will be spectral energy at relatively high harmonics of 1f.sub.c, for example, at 10f.sub.c. In FIG. 2B, in the immediate vicinity of 1f.sub.c, the reference window will capture a component of EMI having amplitude A1. In the vicinity of the third harmonic 3f.sub.c, an EMI component of amplitude A3 will be present, and so forth.
Returning now to FIG. 1, it is known in the art to provide an EMI-reducing module 120 that includes low pass filters 130, and/or ferrite beads or other energy absorbing components 140. Such low pass filters and energy absorbing components may be useful in reducing differential mode and common mode EMI, respectively.
Low pass filters 130 may be implemented with conventional components such as operational amplifiers, resistors, capacitors, inductors. These filters typically have a cutoff frequency of about twice the fundamental frequency, or about 10 MHz for a 5 MHz panel clock frequency. As such, the lowpass filters attenuate some high frequency components from the panel clock and data bus signals, and can reduce EMI to a limited degree.
It is apparent from FIG. 2B that if all frequency components higher than 1f.sub.c were removed by low pass filters 130, relatively little EMI energy would remain within the reference window bandwidth as it sweeps higher than 1f.sub.c. Unfortunately, however, such excessive low pass filtering would slow the panel clock and pixel data signals, compromising the ability of the flat panel to provide a meaningful display.
Further, low pass filtering can only be truly effective where the EMI signals are in a differential mode, e.g., where EMI is present on the panel clock and/or data bus signal wires, but is not present on the system ground 150. Those skilled in the art will appreciate that reducing the effective impedance of the system ground return 150 will reduce the EMI voltage drop resulting from EMI signal currents. Reducing the ground impedance can be a very effective method of reducing EMI.
In some application the EMI is common mode, e.g., carried on the panel clock wire, the data bus wire(s), and also on ground. It is known in the art to reduce common mode EMI by placing energy dissipating elements such as ferrites 130 in close proximity to such wires. The dissipating elements absorb the electromagnetic energy from the EMI, converting the energy into heat. The use of ferrite beads, cores, or other dissipating elements can effectively contain EMI to limited areas within an enclosure. However, the amount of EMI attenuation is relatively small, and other EMI-reducing techniques must also be used.
It is also known in the art to surround EMI-radiating equipment with a metal shield 160 that confines the radiation to the equipment. Shielding can be effective but can be costly and add to the system size. Further, effective shielding may impair system cooling, for example by reducing or eliminating ventilation openings.
A somewhat more sophisticated approach to reducing EMI is to replace the crystal controlled main oscillator 40 and frequency divider 50 within the display generator with a frequency slewable clock unit 170. More specifically, the output signal from a sweep generator 180 is coupled to the input of a voltage controlled oscillator 190. The output from the voltage controlled oscillator 190 is then presented as the input to the timing generator 60.
The purpose of the substitute clock unit 170 is to rapidly change the frequency provided by the timing generator 60. Sufficiently rapid changes reduce the amount of time that frequency components fall within the narrow EMI-compliance reference bandwidth. Since EMI measurements represent an integration of spectral energy over time, reducing the time that spectral components fall within the reference bandwidth will reduce their EMI contribution.
Unfortunately it is difficult to implement clock unit 170 as most modern digital clock circuity is crystal controlled, and thus not appreciably slewable. Generally implementation of the sweep generator and VCO requires a customized integrated circuit, and thus represents additional cost to manufacture the display generator.
What is needed is a technique for reducing differential mode and common mode EMI in a display system that effectively reduces EMI without significant impact upon display performance. Preferably such technique should be capable of implementation using off-the-shelf components that do not add significantly to the cost of manufacturing a video display system. Further, such technique should not add significantly to the package size of the video display system, and should not hamper system cooling.
The present invention discloses such a technique.